1. Technical Field
The present invention relates to spark plugs for internal combustion engines and methods of manufacturing the spark plugs.
2. Description of the Related Art
In a spark plug for an internal combustion engine, for the purpose of extending the service life of the spark plug, a refractory metal material (e.g., a tungsten alloy) is generally used for making a center electrode of the spark plug. Here, the term “refractory metal material” denotes a metal material having a high melting point.
However, a refractory metal material is generally expensive. Therefore, for reducing the manufacturing cost, it is possible to make a base portion of the center electrode with an inexpensive metal material (e.g., a nickel alloy) and a distal portion of the center electrode, which is particularly easy to be consumed in the center electrode, with a refractory metal material. In this case, since the refractory metal material generally has a low coefficient of thermal expansion, it is important to reduce thermal stress induced in the center electrode due to the difference in coefficient of thermal expansion between the refractory metal material and the inexpensive metal material of which the base portion is made.
For example, Japanese Unexamined Patent Application Publication No. H7-037673 discloses a spark plug in which the center electrode has its base portion made of a nickel alloy and its distal portion (or discharge chip) made of a tungsten alloy. The distal portion is joined to a distal end of the base portion by laser welding to form a weld therebetween. More specifically, the weld is made up of those parts of the base portion and the distal portion which are molten and mixed together during the laser welding and solidified after the laser welding. Moreover, the weld is formed, along the outer periphery of the interface between the base portion and the distal portion, into an annular shape.
However, the spark plug disclosed in the above patent document involves the following problems.
In the spark plug, the base portion and the distal portion of the center electrode are joined to each other by only the annular weld formed along the outer periphery of the interface between the base portion and the distal portion. That is, on the radially inside of the annular weld, there exists a non-joined region where the base portion and the distal portion are not joined to each other. Consequently, concentration of thermal stress may occur at the boundary between the weld and the non-joined region, thereby causing a joining fault, such as cracks, to occur at the boundary.
In addition, one may consider forming the weld over the entire interface between the base portion and the distal portion, thereby eliminating the non-joined region. However, in this case, since the melting point of the base portion is lower than that of the distal portion, the base portion may be excessively molten during the laser welding, causing the molten material of the base portion to be scattered and volatilized.